1. Field of the Invention
The present invention generally relates to an apparatus and method for transmitting and receiving information in a mobile communication system. More particularly, the present invention relates to an apparatus and method for transmitting and receiving information through a reverse control channel in a mobile communication system for providing packet data.
2. Description of the Related Art
Conventionally, mobile communication systems have been divided into systems for supporting only a voice service, only a data service and both the voice service and the data service. In these systems, the most widely used communication scheme is Code Division Multiple Access (CDMA). The systems using the CDMA are referred to as the CDMA mobile communication systems. Among the CDMA mobile communication systems, the system for supporting only the voice service is a system of the Interim Standard (IS-95). The CDMA system for supporting both the voice service and the data service is a CDMA 2000 system. For example, a mobile communication system for providing only high-rate packet data is a High Rate Packet Data (HRPD) mobile communication system based on the CDMA. In the mobile communication systems, both forward and reverse link transmissions are present. Herein, a forward link is a link from a base station to a mobile station and a reverse link is a link from a mobile station to a base station.
The forward and reverse link transmissions in the HRPD mobile communication system will be described. The HRPD mobile communication system exploits Time Division Multiple Access (TDMA) as a multiple access scheme and exploits Time Division Multiplexing/Code Division Multiplexing (TDM/CDM) as a multiplexing scheme. The forward link transmission is performed after data to be transmitted in the forward link is set in the HRPD system. Alternatively, when data is transmitted in the reverse link in the HRPD system, multiple mobile stations simultaneously transmit data in the reverse link.
To support high-speed data communication in the forward and reverse directions in the CDMA mobile communication system such as the CDMA 2000 or HRPD system, the mobile station transmits a Data Rate Control (DRC) channel carrying information regarding a transmission rate of data to be received in the forward direction and transmits a Reverse Rate Indicator (RRI) channel carrying information about a traffic channel to be transmitted in the reverse direction. The base station and the mobile station exchange the control information, thereby smoothly performing data communication.
On the other hand, the mobile communication system divides one encoded transmission packet into multiple subpackets to transmit the subpackets in a subpacket unit using a Hybrid Automatic Request (HARQ) transmission scheme. Thus, a receiving side decodes received subpackets, thereby increasing data transmission efficiency. However, subpacket index information indicating a position of an associated subpacket of the entire packet is required to decode the received subpackets. That is, when the HARQ transmission scheme is exploited, data rate information should be transmitted in every subpacket. Consequently, the subpacket index and the data rate information of a data rate indicator channel should be simultaneously transmitted.
In an example of a system using the data rate indicator channel, the RRI channel is used for the reverse link of CDMA 1x Evolution Data Only (1xEV-DO) Revision A (hereinafter, referred to as “1xEV-DO”). In 1xEV-DO, a pilot channel, a control channel including an RRI, and a data channel are transmitted in parallel. The channels are spread by different Walsh codes, and are transmitted after time division. Transmit power of a signal to be transmitted on each channel is adjusted by a gain of each channel defined as a relative value to a pilot channel. In the reverse link of 1xEV-DO, a value between 0 and 15 is transmitted on the RRI channel according to payload size, so that a receiver is notified of a data rate. Furthermore, a Walsh code used to spread an RRI symbol is changed according to a subchannel index so that the subchannel index for the HARQ is transmitted together with the RRI symbol.
The mobile communication system transmits/receives an Acknowledgement/Negative Acknowledgement (ACK/NACK) signal for notification of a reception state for a traffic channel, and transmits a Data Source Control (DSC) channel and so on for supporting a handoff between base stations.
When the information is constantly transmitted, interference may occur due to the signals. As the interference increases, the total capacity decreases. This problem becomes severe as the number of mobile stations for receiving service from one base station increases. In this case, the reverse capacity significantly decreases. Consequently, a reverse transmission rate is limited.
To remove the reverse interference of the mobile station, a method using Discontinuous Transmission (DTX) mode for control channels has been proposed. This method will be described with reference to FIG. 1.
FIG. 1 is a timing diagram illustrating the DTX mode for reverse control channels in the 1xEV-DO mobile communication system in which the DTX is performed.
FIG. 1 illustrates an example in which the DTX of reverse traffic is performed in a 4-slot unit. A reverse traffic channel 141 transmits data at a predetermined transmission rate when the data to be transmitted in the reverse link is present in the mobile station. The mobile station transmits RRI information on an RRI channel to give notification of the transmission rate of the reverse traffic. At this time, the DTX is performed in a 2-slot unit in the 1xEV-DO system. As illustrated in the lower section of FIG. 1, a control channel is generated in which a transmission interval and a non-transmission interval are repeated in the 2-slot unit. The mobile station transmits the RRI information on the RRI channel in the transmission interval and does not transmit the RRI information in the non-transmission interval. That is, the RRI channel transmits the RRI information to give notification that a reverse transmission rate is 0 in intervals 131 and 135 in which no reverse traffic is transmitted. The RRI channel indicates a transmission rate of data to be transmitted in the reverse link in intervals 132 and 134 in which the reverse traffic is transmitted.
The reason why the notification that the reverse transmission rate is 0 is given is as follows.
In the communication system that uses a packet transmission scheme, a transmission data rate is adjusted according to presence of data or traffic data that can be discontinuously transmitted. If a transmitter does not report the presence of data transmission to a receiver, the receiver should verify the presence of data in every time interval in which the packet transmission is possible. When data is not transmitted to reduce an unnecessary load of the receiver, a data indicator channel is transmitted at a null rate to notify that a packet to be transmitted on a data channel is absent. This occurs where an RRI symbol transmits 0.
Referring to FIG. 1, no RRI information is transmitted in the non-transmission interval since an interval 133 in which traffic is transmitted in the reverse link is mapped to the DTX mode. In the 1xEV-DO system, the mobile station transmits DRC information for indicating a transmission rate for forward reception to the base station through the DRC channel. The system of FIG. 1 supports the DTX mode. The DRC channel transmits the DRC information in intervals 121 and 122, but does not transmit it in the non-transmission interval.
The DSC channel for the handoff is transmitted together with an ACK channel (ACKCH) within one slot through time division. That is, DSC information is transmitted in a half slot of one slot and an ACK signal is transmitted in the remaining half slot. The ACK signal and the DSC information are transmitted according to the transmission interval and the non-transmission interval. Also, an interval of the reverse pilot channel is divided into the transmission interval and the non-transmission interval. That is, an interval 103 is the transmission interval in which a reverse pilot is transmitted and an interval 104 is the non-transmission interval in which no reverse pilot is transmitted. The reverse pilot may also be transmitted in the non-transmission interval only when reverse data is transmitted, such as, the reverse data is present as in the interval 141.
As described above, the RRI information indicates a transmission rate of traffic to be transmitted in the reverse direction. Thus, the RRI information is used to demodulate and decode data. However, when the RRI is partially transmitted also in the interval when the reverse traffic channel is transmitted in the DTX mode, the following problems occur.
First, when the RRI information is discontinuously transmitted even in the interval when traffic is transmitted on the reverse traffic channel, it is difficult for necessary control information to be extracted without error when a traffic channel of an associated interval is received, demodulated, and decoded using the discontinuously transmitted RRI information. That is, traffic demodulation and decoding performances may be degraded due to RRI error.
Second, when an error occurs in the RRI information, a retransmission ratio of reverse traffic increases. Consequently, loss due to the retransmission of reverse traffic is greater than the gain obtained by discontinuously transmitting a reverse control channel. When reverse traffic is absent as in the intervals 103 and 104 in which the pilot is discontinuously transmitted, the discontinuous transmission of the RRI does not become a large problem. Because the RRI information is not used to recover a reverse traffic channel when the reverse traffic channel is not transmitted, a method for transmitting a reverse control channel to effectively demodulate and decode reverse traffic is required.
Third, because the RRI channel transmits only information for packet recovery in the receiver, power used for the RRI channel and interference serve as overhead in the overall system and therefore system capacity decreases. Because an amount of data to be transmitted on the RRI channel is constant regardless of a packet data rate, overhead due to the RRI channel relatively increases when an amount of traffic to be transmitted in the reverse direction is small, such as, a data rate is low.
Fourth, the battery use time is reduced as transmit power due to the RRI channel increases in the case of the reverse link of the 1xEV-DO system. For example, the RRI channel is continuously transmitted, despite a low transmission data rate and a low ratio of an actual packet transmission time to a total communication time when Voice over Internet Protocol (VoIP) is used. Thus, power consumption due to the RRI channel relatively increases. This causes the effective use time of the mobile station to be reduced.
Accordingly, there is a need for an improved system and method for transmitting a reverse control channel that can effectively demodulate and decode reverse traffic when the reverse traffic is present in a mobile communication system for supporting a discontinuous transmission.